Managing Cooling Tower Cycles of Concentration...Cycles of Concentration Definition. •Cycles of...
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Managing Cooling Tower Cycles of Concentration
Nick McCall, P.E., Woodard & Curran
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Introduction
Chiller & Cooling Best Practices® Magazine
Managing Cooling Tower Cycles of Concentration
Sponsored by
• Technical Manager,
Woodard & Curran
• Utilities engineer since 2008
• Handle utilities installations
including chillers, air
compressors, dryers, boilers,
and cooling towers as well as
supporting utilities for
manufacturing equipment
About the Speaker
Sponsored byNick McCall, P.E.Woodard & Curran
Cooling Tower Function Overview
• Cooling towers dissipate heat from processes, typically using water as the medium.
• Heat transfer due to conduction is approximately 20-30%, remaining 70-80% is the result of evaporative cooling.
• Rate of evaporation is approximately 1-2% of the recirculating water flow depending on temperature delta across the tower.
• Water lost due to evaporation, drift, system leaks, and blowdown is replaced via makeup water.
Cooling Tower Function Overview
Image courtesy SPX
Makeup Water Equation
• Cooling tower operation is described by the relationship between evaporation,
blowdown, and makeup. Note: loss due to drift/system leaks is included in the
blowdown flow rate.
𝑀 = 𝐵 + 𝐸
𝑀 = 𝑚𝑎𝑘𝑒𝑢𝑝 𝑤𝑎𝑡𝑒𝑟 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒𝐵 = 𝑏𝑙𝑜𝑤𝑑𝑜𝑤𝑛 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒𝐸 = 𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒
Cycles of Concentration Definition
• Cycles of concentration (COC) describes the relationship between the
makeup water rate and the blowdown rate.
• It is also a measure of the total amount of minerals concentrated in the
cooling tower water relative to the amount of minerals in the incoming
makeup water.
• Higher COC is indicative of higher water use efficiency.
• Most tower systems operate with a COC between 3 and 10, 10 being more
efficient.
• COCs tend to range between 5 and 7 due to cost efficiency.
Cycles of Concentration Equation
• COC can be calculated from makeup and blowdown flow rates as below:
𝐶 =𝑀
𝐵
𝐶 = 𝐶𝑂𝐶𝑀 = 𝑚𝑎𝑘𝑒𝑢𝑝 𝑤𝑎𝑡𝑒𝑟 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒𝐵 = 𝑏𝑙𝑜𝑤𝑑𝑜𝑤𝑛 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒
Cycles of Concentration Via Conductivity
• COC can also be determined via water analysis.
• This can be done by measuring the conductivity of the incoming makeup water and the recirculating cooling tower water (the recirculating water conductivity will be the same as the blowdown water.)
𝐶 =𝐵𝑐𝑜𝑛𝑑𝑀𝑐𝑜𝑛𝑑
𝐶 = 𝐶𝑂𝐶𝐵𝑐𝑜𝑛𝑑 = 𝑏𝑙𝑜𝑤𝑑𝑜𝑤𝑛 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦𝑀𝑐𝑜𝑛𝑑 = 𝑚𝑎𝑘𝑒𝑢𝑝 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦
• Conductivity is commonly used to estimate COC.
Cycles of Concentration Management
• Simple rule: to increase COC, decrease blowdown; to decrease COC,
increase blowdown.
• COC can be adjusted to allow for lower rates of water use, corresponding to
lower rates of water chemical treatment use.
Blowdown Required Equation
• Water evaporation loss can be used to determine the blowdown rate needed
to operate at a given COC. The relationship is as below:
𝐵 =𝐸
(𝐶 − 1)
𝐵 = 𝑏𝑙𝑜𝑤𝑑𝑜𝑤𝑛 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒𝐸 = 𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒
𝐶 = 𝐶𝑂𝐶
Evaporation Rate Rule of Thumb
• Evaporation rate typically is 1% of the recirculation rate for every 10 deg F
temperature drop across the tower (rule of thumb.)
• Newer towers can have 0.75% of the recirculation rate for every 10 deg F
drop.
Scale
• Scale is formed from minerals dissolved in the makeup water.
• Scale is a byproduct of water evaporation in the tower, making the concentration of the minerals higher in the remaining water.
• Minerals eventually come out of solution and deposit on surfaces in contact with the cooling tower water.
• Common scaling minerals include calcium carbonate, calcium phosphate, calcium sulfate, and silica (magnesium sulfate is also possible under certain conditions.)
• Most calcium and magnesium salts are more soluble in cold water than hot ( “reverse solubility.”)
• Other salts such as silica are more soluble in hot water than in cold.
• Water temperature increase as the water moves through the system causes calcium and magnesium scale to form.
• Deposits can form anywhere in the system, but they are most likely on hot surfaces such as heat exchangers.
• Silica tends to form in the coldest parts of the system (cooling tower fill.)
Scaling Potential and Cycles of Concentration Limits
• Scaling potential is determined by the maximum solubility limit for dissolved minerals that can form scale given a particular set of conditions.
• Minimizing blowdown requires dissolved mineral levels to be maintained as close as possible to these maximum solubility levels (also referred to as total dissolved solids or TDS), and it is controlled by maintaining COC for the system at a level that is equal to the lowest COC allowable for the lowest solubility salt.
• Typically calcium carbonate or calcium phosphate, but it may also be silica under certain conditions.
• COC can be increased through proper cooling water treatment.
• Equations that follow are based on rules of thumb to establish rough limits on COC. Final operating COC will be dependent on system conditions and water treatment chemicals used in system.
Calcium Carbonate Scale
• Calcium carbonate scale is formed when calcium bicarbonate breaks down.
• Severity of scale depends on calcium level, bicarbonate alkalinity level, and water temperature in the system.
𝐶 =110,000
𝑇𝐴 𝑥 𝑀𝐶𝑎
𝐶 = 𝐶𝑂𝐶𝑇𝐴 = 𝑇𝑜𝑡𝑎𝑙 𝑎𝑙𝑘𝑎𝑙𝑖𝑛𝑖𝑡𝑦 𝑎𝑠 𝐶𝑎𝐶𝑂3 𝑖𝑛 𝑚𝑎𝑘𝑒𝑢𝑝 𝑖𝑛 𝑝𝑝𝑚
𝑀𝐶𝑎 = 𝐶𝑎𝑙𝑐𝑖𝑢𝑚 ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 𝑎𝑠 𝐶𝑎𝐶𝑂3 𝑖𝑛 𝑚𝑎𝑘𝑒𝑢𝑝 𝑖𝑛 𝑝𝑝𝑚
Calcium Phosphate Scale
• Calcium phosphate scale occurs when calcium hardness reacts with phosphate in the system.
• Calcium hardness must be sufficiently high, and orthophosphate must be present and higher than 10 ppm in the cooling water.
• Calcium phosphate scaling potential can be roughly predicted by the below:
𝐶 =(105 𝑥 (9.8 − 𝐵𝑝𝐻))
𝑀𝐶𝑎
𝐶 = 𝐶𝑂𝐶𝐵𝑝𝐻 = 𝑏𝑙𝑜𝑤𝑑𝑜𝑤𝑛 𝑝𝐻
𝑀𝐶𝑎 = 𝐶𝑎𝑙𝑐𝑖𝑢𝑚 ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 𝑎𝑠 𝐶𝑎𝐶𝑂3 𝑖𝑛 𝑚𝑎𝑘𝑒𝑢𝑝 𝑖𝑛 𝑝𝑝𝑚
Calcium Sulfate Scale
• Calcium sulfate scale occurs when calcium hardness reacts with sulfate in the water.
• Scaling potential can be roughly predicted by the below:
𝐶 =1,250,000
𝑀𝐶𝑎𝑥𝑀𝑆𝑢
𝐶 = 𝐶𝑂𝐶𝑀𝐶𝑎 = 𝐶𝑎𝑙𝑐𝑖𝑢𝑚 ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 𝑎𝑠 𝐶𝑎𝐶𝑂3 𝑖𝑛 𝑚𝑎𝑘𝑒𝑢𝑝 𝑖𝑛 𝑝𝑝𝑚
𝑀𝑆𝑢 = 𝑠𝑢𝑙𝑓𝑎𝑡𝑒 𝑎𝑠 𝑆𝑂4 𝑖𝑛 𝑚𝑎𝑘𝑒𝑢𝑝 𝑖𝑛 𝑝𝑝𝑚
Silica Scale
• Silica scale can occur when the maximum solubility for silica is exceeded in the cooling water. A conservative value for the solubility limit is 150 PPM as SiO2. However, the solubility limit for silica is dependent on pH and temperature. The limit ranges between 150-180 ppm in the temperature ranges typically encountered in tower systems (80 deg F to 130 deg F.) As pH increases, the solubility limit goes up. For example, the limit in a tower system with a pH of 9.0 would be approximately 250 ppm. Using 150 ppm as the limit, the below can be used to determine silica scaling potential:
𝐶 =150
𝑀𝑆𝑖
𝐶 = 𝐶𝑂𝐶150 = 𝑎𝑠𝑠𝑢𝑚𝑒𝑑 𝑠𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦 𝑙𝑖𝑚𝑖𝑡 𝑓𝑜𝑟 𝑠𝑖𝑙𝑖𝑐𝑎 𝑖𝑛 𝑝𝑝𝑚
𝑀𝑆𝑖 = 𝑠𝑖𝑙𝑖𝑐𝑎 𝑎𝑠 𝑆𝑖𝑂2 𝑖𝑛 𝑚𝑎𝑘𝑒𝑢𝑝 𝑖𝑛 𝑝𝑝𝑚
Maximum Cycles of Concentration
• Lowest calculated COC from the above equations is the controlling factor for
operating cooling towers.
• Material with the lowest COC will be the first to precipitate out of solution,
forming scale deposits in the system.
• System COC must be kept lower than this value for the above salts.
• Water treatment can be used to allow for higher COC values, increasing the
water use efficiency of the system.
Example
• 𝑅 (𝑟𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒) = 3500 𝐺𝑃𝑀
• 𝑇 (𝑑𝑒𝑙𝑡𝑎 𝑎𝑐𝑟𝑜𝑠𝑠 𝑡𝑜𝑤𝑒𝑟) = 13.5° 𝐹
• 𝑀𝐶𝑎 = 255 𝑝𝑝𝑚 𝑎𝑠 𝐶𝑎𝐶𝑂3• 𝑇𝐴 = 155 𝑝𝑝𝑚 𝑎𝑠 𝐶𝑎𝐶𝑂3• 𝑃ℎ𝑜𝑠𝑝ℎ𝑎𝑡𝑒 = 3 𝑝𝑝𝑚 𝑎𝑠 𝑃𝑂4• 𝐵𝑝𝐻 = 8.5
• 𝑀𝑆𝑢 = 165 𝑝𝑝𝑚 𝑎𝑠 𝑆𝑂4• 𝑀𝑆𝑖 = 5 𝑝𝑝𝑚 𝑎𝑠 𝑆𝑖𝑂2
Example
• 𝐸 = 𝑅 𝑥 0.01 𝑥𝑇
10
• 𝐸 = 3500 𝑥 0.01 𝑥13.5
10= 47.25 𝐺𝑃𝑀
• Calcium carbonate: 𝐶 =110,000
𝑇𝐴 𝑥 𝑀𝐶𝑎=
110,000
155 𝑥 255= 1.67
• Calcium phosphate: phosphate level < 10 ppm, N/A
• Calcium sulfate: 𝐶 =1,250,000
𝑀𝐶𝑎 𝑥 𝑀𝑆𝑢=
1,250,000
255 𝑥 165= 5.45
• Silica: 𝐶 =150
𝑀𝑆𝑖=
150
5= 30
• Calcium carbonate returns the lowest value, therefore 𝐶 = 1.67
• 𝐵 =𝐸
(𝐶−1)=
47.25
(1.67−1)= 70.52 𝐺𝑃𝑀
• 𝑀 = 𝐵 + 𝐸 = 47.25 + 70.52 = 117.77 𝐺𝑃𝑀
Summary
• Cycles of concentration (COC) describes the relationship between the makeup water and blowdown, both in rate and mineral concentration.
• Higher COC is indicative of higher water/chemical use efficiency. Most systems operate between 3-10 COC.
• Simple rule: to increase COC, decrease blowdown; to decrease COC, increase blowdown.
• COC can be adjusted to control scaling and allow for lower rates of water/chemical use.
• Scaling potential can be estimated based on makeup water quality parameters.
• COC can be increased through proper cooling water treatment.
Nick McCall, PE
About the Speaker
Sponsored by
• Senior Manager of Technical
Services, SPX Cooling
Technologies, Inc.
•30 year of cooling tower
experience
•Secretary of ASHRAE’s
Technical Committee for
Cooling Towers and
Evaporative Condensers
(TC8.6)
Mark PfeiferSPX Cooling Technologies, Inc.
Cycles of Concentration –A Manufacturers Perspective
May 20, 2021
1. Why Evaporative Cooling?
2. Why should I Maximize Cycles of Concentration?
3. Other water saving methods
4. How manufacturers can help
© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Agenda
1. Why Evaporative Cooling?
2. Why should I Maximize Cycles of Concentration?
3. Other water saving methods
4. How manufacturers can help
© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Agenda
The “Cooling” in Cooling Towers
May 20, 2021 31
Sensible (aka dry cooling)
• Sensible Cooling of 1 lb of water 1ºF rejects 1 btu.
• Dry Bulb temperature is the driving force
• Hard to cool 95º water with 95º air
• Example: Car radiator
Latent (aka evaporative cooling)
• Evaporating that same 1 lb of water rejects 1,000 btu!
• Example: Perspiration evaporating
Copyright © 2021 SPX Cooling Technologies, Inc. All rights reserved.
The “Cooling” in Cooling Towers
May 20, 2021 32
Image courtesy Trane Technologies
Evaporative cooling
• Can cool water approaching the wet bulb temperature
• providing colder water to process
• Providing additional system efficiency
Copyright © 2021 SPX Cooling Technologies, Inc. All rights reserved.
1. Why Evaporative Cooling?
2. Why should I Maximize Cycles of Concentration?
3. Other water saving methods
4. How manufacturers can help
© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Agenda
34© 2021 SPX Cooling Technologies, Inc. All Rights Reserved May 20, 2021
Water Use in Evaporative Equipment
Cycles of Concentration
▪ Number of times the dissolved solids in a
particular volume of water are concentrated
through evaporation
▪ Regulated by adjusting the blowdown rate
0
2
4
6
8
10
12
14
16
2 3 4 5 6 7 8 9 10
Blo
wdow
n (
gpm
)
Cycles of Concentration
Water Quality – Scale
355/20/2021© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
1. Why Evaporative Cooling?
2. Why should I Maximize Cycles of Concentration?
3. Other water saving methods
4. How manufacturers can help
© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Agenda
Condensate Recovery
May 20, 2021 37
Condensate from cooling coilsOptimal use of condensate as cooling tower makeup:
• Condensate production occurs when tower is active
• No storage tank
• No additional water treatment
• Reduced blowdown
Reference: “Quality of Condensate From Air-Handling Units,” ASHRAE Journal, December 2016, Glawe and Wooten
© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Condensate Recovery
May 20, 2021 38
Potential ANNUAL condensate volume from air handlers per cfm of airflow in
different climates:
• Athens, GA 12.5 gal
• Houston, TX 22.4 gal
• Boston, MA 4.5 gal
• Sacramento, CA 1.3 gal
• Denver, CO 0.5 gal
Reference: “Capturing Condensate by Retrofitting AHU’s,” ASHRAE Journal, January 2010, Lawrence, Perry
and Dempsey
© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Grey and Reclaimed Water Use
Grey: household wastewater (as from a sink or bath) that does not contain serious contaminants (as from toilets)(1)
Reclaimed: Waste water treatment plant effluent, generally having been the subject of significant water treatment for removal of “nutrients, toxic compounds, (TSS) [total suspended solids], and organics.”(2)
(1) https://www.merriam-webster.com/dictionary
(2) Florida Water Resources Journal, “Comparison of Water Quality Parameters from South Florida Wastewater Treatment Plants”, Bloetscher & Gokgoz, 6/01
5/20/2021 39© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Fluid Cooler Products – Dry Capacity
May 20, 2021
• Products available to meet varying dry operation goals
• Water savings / water treatment • Basin freeze prevention / wintertime
operation• Operational flexibility / redundancy
Coil Surface Area Dry Capacity
Airflow Dry Capacity
Coil Surface Area Cost
Fluid Cooler
Type First Cost
Dry
Capacity
Hybrid
Crossflow$$$ --
Hybrid
Counterflow$$ -
Bare Coil
only
Counterflow
$$$ +
Finned Coil
only
Counterflow
$$$$ ++
Adiabatic
Fluid Cooler$$$$$ +++
© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
1. Why Evaporative Cooling?
2. Why should I Maximize Cycles of Concentration?
3. Other water saving methods
4. How manufacturers can help
© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Agenda
Tools - Water Guidelines
May 20, 2021 42
Reference cooling tower
manufacturer’s guidelines to
help monitor the quality of
your water
Provides limits to minimize:
1. Scale
2. Corrosion
3. Deposits
4. Biological Growth
© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Tools - Water Use Calculator
May 20, 2021 43© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Tools - Water Use Calculator
May 20, 2021 44© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Tower Options – Water Management Accessories
• Conductivity controllers
• Blowdown water valves and meters
• Makeup water valves and meters
• Water level controllers
• Basin filtration
455/20/2021© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Tower Options - Sweeper Piping
May 20, 2021 46
• Provides piping system in collection
basin with nozzles to move debris and
sediment from the basin
Counterflow Basin Crossflow Basin
Sweeper Inlet
Sweep Outlet
• Outlet is connected to centrifugal
separator or filtration system
© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
Thank You. Questions?
[email protected] – Technical Services
47© 2021 SPX Cooling Technologies, Inc. All Rights Reserved
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Managing Cooling Tower Cycles of Concentration
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